The present application relates generally to the field of liquid crystal displays including a backlight. More particularly, the application relates to a liquid crystal display including a dual mode backlight that is compatible with night vision imaging system (NVIS) Class A requirements.
Liquid crystal displays (LCDs) are desirable for use as display means in air vehicle cockpits. LCDs provide high definition, bright displays readable under both low light and full sunlight conditions. LCD producers are facing increasing demand for LCDs having premium features. One example of a premium feature is night vision imaging system (NVIS) compatibility.
A night vision imaging system allows the pilot or operator to use night vision imaging goggles while flying the aircraft. Night vision imaging goggles are sensitive to light primarily in the red to near-infrared wavelengths, approximately 610 nanometers (nm) to 930 nm in the color spectrum (the night vision spectrum). Night vision imaging goggles allow the pilot to view outside conditions in low light using light in the night vision spectrum, allowing aircraft operation during nighttime conditions.
However, it is important to ensure that aircraft LCD display systems provided in an aircraft are compatible with the use of night vision imaging goggles. If the light out from the LCD display is within the night vision spectrum and therefor viewable by the night vision goggles, the light output from the display can be seen reflecting off the canopy, degrading goggle performance and pilot vision. Incompatible lights make the outside scene less visible with the goggles. Additionally, changing illumination can affect visual acuity. Finally, external incompatible light from the ambient environment can result in “washout” or halo effects, when using the goggles. This could result in glare, flash blindness, and afterimage for the pilot.
A NVIS compatible display provides light output that does not include light having a wavelength viewable by night vision imaging goggles. The NVIS compatible display must provide light outside the night vision spectrum with sufficient luminance to be visible. It is also desirable to have a means to reduce the luminance output of the LCD displays compared with daytime operation to reduce overall visibility and canopy reflection. However, the same display must continue to function as a standard display during daytime operation, wherein bright illumination is required so that the display is visible even in bright sunlight.
In addition, there are two types of NVIS compatibility, Class A compatibility and Class B compatibility. The NVIS Class B response curve is from approximately 630 nm to approximately 910. The NVIS Class A response curve is from approximately 610 nm to approximately 910 nm. The Class A response curve reaches lower into the color spectrum, requiring more filtering than is required for Class B compatibility. Filtering more of the color spectrum can be accomplished using an NVIS Class A filter positioned between a white light source and the LCD stack to filter light above 610 nm.
Traditionally, cockpit displays have relied on light source dimming combined with filtering to minimize emissions. Although using a NVIS filter will filter the required spectrum of light, it has the effect of filtering red content from the display image. Filtering has the effect of reducing emissions between 630 and 900 nm while transmitting light within the visible range. Thus, the red in the display image is reduced during daylight operation, and the display is no longer a full color display. The red color in an image appears to be less saturated or more orange than the “true” image. Lack of a full color display can be problematic in a display wherein color is used as an indication means. An example can be a weather condition display wherein red colors indicate severe weather.
Therefore, there is a need for a full color display that is NVIS Class A compatible. Further, there is a need for such a display wherein the red emission is not compromised during daylight viewing, such that the display is a full color display.